Pad conditioning force modeling to achieve constant removal rate

ABSTRACT

A method and apparatus for conditioning a polishing pad in a CMP system is provided. In one embodiment, a method for conditioning a polishing pad includes applying a down force to the conditioning disk that urges the conditioning disk against the polishing pad, measuring a torque required to sweep the conditioning disk across the polishing pad, determining a change in down force by comparing the measured torque to a model force profile (MFP), and adjusting the down force that the conditioning disk applies against the polishing pad in response to the determined change.

BACKGROUND

1. Field of the Invention

Embodiments of the present invention generally relate a method forconditioning a polishing surface in an electrochemical mechanicalprocessing system.

2. Description of the Related Art

During the manufacture of semiconductor devices, layers and structuresare deposited and formed on a semiconductor substrate by variousprocesses. Chemical mechanical polishing (CMP) is a widely used processby which a polishing pad in combination with a polishing solutionremoves excess material in a manner that planarizes the substrate ormaintains flatness for receipt of a subsequent layer. Over time, theeffectiveness of the polishing pad diminishes as pressure, friction, andheat combine with particulate matter from processing slurries, materialsremoved from the substrate (or from the pad itself), and the like, toform a hard, relatively smooth surface on the pad. This effect istypically called “glazing.” In order to improve the effectiveness of thepolishing pad after glazing has occurred, the polishing pad may beperiodically conditioned. Pad conditioning generally involves scouringthe polishing pad with an abrasive conditioning disk to remove anyaccumulated polishing by-products on the pad surface and/or to refreshthe surface of the polishing pad. The conditioning of the polishing padsurface may be performed prior to polishing with a new polishing pad,during the polishing process to maintain and/or enhance surfaceroughness and removal rate of the polishing pad surface, orpost-processing to prepare the polishing pad surface for a new substrateto be polished.

It is generally known that the effectiveness of the conditioning diskdeclines over time due to disk and pad wear. As a result, theeffectiveness of the polishing pad drifts over time, producingnon-uniform results from substrate to substrate. Thus, there is a needfor an improved method for conditioning a polishing pad to improvepolishing pad performance over the life of the conditioning disk.

SUMMARY OF THE INVENTION

Embodiments of the invention provide methods for conditioning apolishing pad. In one embodiment, a method for conditioning a polishingpad includes applying a down force to the conditioning disk that urgesthe conditioning disk against the polishing pad, measuring a torquerequired to sweep the conditioning disk across the polishing pad,determining a change in down force by comparing the measured torque to amodel force profile (MFP), and adjusting the down force that theconditioning disk applies against the polishing pad in response to thedetermined change.

In another embodiment, a method for conditioning a polishing padincludes applying a down force to urge a conditioning disk against apolishing pad, measuring a frictional force generated by theconditioning disk contacting the polishing pad, comparing the measuredfrictional force to a model force profile (MFP) to determine a change indown force, and adjusting the applied down force in response to thecomparison between the measured frictional force and the MFP.

In yet another embodiment, an apparatus conditioning a polishing padwith a conditioning disk includes a platen adapted to support thepolishing pad, a conditioning head adapted to retain the conditioningdisk, a down force actuator operable to move the conditioning head in amanner that conditioning head applies a down force against the polishingpad, an arm coupled to the conditioning head to support the conditioninghead above the platen, a sweep actuator coupled to the arm and operableto sweep the conditioning head across the platen, a sweep torque sensoroperable to measure a torque required to sweep the conditioning headacross the platen when the conditioning disk is in contact withpolishing pad.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings.

FIG. 1 is a sectional view of an exemplary CMP system which may be usedto practice embodiments of the invention.

FIG. 2 is a top view of the CMP system of FIG. 1.

FIG. 3 is a flow diagram of one embodiment of a method of padconditioning.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

It is to be noted, however, that the appended drawings illustrate onlyexemplary embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

DETAILED DESCRIPTION

Embodiments of the invention are directed towards the performance of aCMP polishing pad. Embodiments of the disclosure provide a method andapparatus for controlling a CMP polisher based on a relationship betweenconditioning arm sweep torque and friction between the polishing pad andconditioning disk. Embodiments include a method for adjusting adown-force (down force) applied by a conditioning disk to a polishingpad based on a difference between a measured force and a modeled forceprofile to improve processing results. It is contemplated that aspectsof the present disclosure that enable use of conditioner sweep torque tomonitor polisher performance may be applied in real time and/or when thepolishing pad is being conditioned while a substrate is being polished(i.e., in-situ conditioning), when the polishing pad is conditionedin-between substrate polishing (i.e., ex-situ conditioning), or anycombination thereof.

FIG. 1 is a sectional view of one embodiment of an exemplary CMP systemaccording to certain aspects of the present disclosure. As shown in FIG.1, the CMP system comprises a polisher 100 having a machine base 130, apolishing fluid delivery arm 190, a polishing pad 104 disposed on aplaten 102, a polishing head 106, a conditioner assembly 122, and acontroller 152. The machine base 130 supports the platen 102, thepolishing fluid delivery arm 190 and the conditioner assembly 122. Theplaten 102 supports polishing head 106.

The polishing head 106 retains and rotatably contacts a substrate 118 tothe polishing pad 104 during processing. The polishing head 106 mayinclude a retaining ring 116 which prevents the substrate 118 frommoving out from under the polishing head 106 during polishing. Thepolishing head 106 may be rotated by a motor 120, thereby rotating thesubstrate 118 against the polishing pad 104 about a central axis D ofthe polishing head 106. A sensor 148 may be utilized to obtain a metricof force required to rotate the substrate 118 against the polishing pad104.

The platen 102 is utilized to rotate the polishing pad 104 duringprocessing such that the polishing pad 104 planarizes (or “polishes”)the surface of the substrate 118 disposed on the pad 104. The polishingpad 104 is a consumable product having a polishing surface and may besecured to the platen 102. The platen 102 and the polishing pad 104 isrotated by a motor 112 coupled to the platen 102 by a shaft 114. Themotor 112 is utilized to move the polishing pad 104 relative to thesubstrate 118 retained in the polishing head 106. In the embodimentdepicted in FIG. 1, the motor 112 rotates the platen 102 in the X-Zplane about a central axis A normal to the platen 102. A sensor 150 maybe utilized to obtain a metric indicative of the force required torotate the platen 102 and polishing pad 104 relative to the substrate118 and/or conditioner assembly 122.

The polishing fluid delivery arm 190 provides a polishing fluid to thesurface of the polishing pad 104 during polishing. The polishing fluidmay comprise an abrasive-containing polishing slurry or may comprise anabrasive-free liquid, which may be reactive.

The conditioner assembly 122 generally includes a conditioning head 108,a shaft 126, and an arm 128. The shaft 126 and arm 128 support theconditioning head 108 above the platen 102. The conditioning head 108retains a conditioning disk 124 which is selectively placed in contactwith the polishing pad 104 to condition the surface of the polishing pad104.

The shaft 126 is disposed through the machine base 130 of polisher 100.The shaft 126 may rotate about an axis B normal to the machine base 130,the rotation facilitated by bearings 132 between the machine base 130and the shaft 126, such that the arm 128 rotates the conditioning head108. In one embodiment, a sweep actuator 144 coupled to the shaft 126may rotate the shaft 126 to urge the arm 128 to sweep the conditioninghead 108 across the polishing pad 104.

The conditioner assembly 122 further includes a sweep torque sensor 146to detect sweep torque required to move the conditioning disk 124 acrossthe surface of the polishing pad 104. In one embodiment, the sweeptorque sensor 146 may be a torque or other force sensor coupled to thesweep actuator 144. In other embodiments, the sweep torque sensor 146may be an electrical current sensor or pressure sensor coupled to thesweep actuator 144. An electrical current sensor may detect changes inthe current drawn by the sweep actuator 144 as the frictional forcesbetween the conditioning disk 124 and the polishing pad 104 change. Apressure sensor may interface with the sweep actuator 144 to detectchanges in the pressure utilized to actuate the sweep actuator 144 asthe frictional forces between the conditioning disk 124 and thepolishing pad 104 change. In another embodiment, the sweep actuator 144may be a direct drive motor configured to provide clean torque feedbackfor measurement and control of the pad conditioner sweep torque. Instill other embodiments, the sweep torque sensor 146 may be any othersensor suitable for providing a metric indicative of the force requiredto move the conditioning disk 124 across the surface of the polishingpad 104.

The conditioning head 108 rotates the conditioning disk 124 about anaxis C disposed normally through the conditioning disk 124. Theconditioning disk 124 is fabricated from a material suitable forconditioning the material of the polishing pad 104. The conditioningdisk 124 may be a brush, polymer or abrasive surface. In one embodiment,the conditioning disk 124 has a surface containing abrasive particlessuch as diamonds or other relatively hard substance.

In one embodiment, a motor 134 is utilized to rotate the conditioningdisk 124 relative to the polishing pad 104. In one embodiment, the motor134 is disposed in a housing 136 at a distal end of the arm 128.

A sensor 138 may detect a torque or rotational force required to rotatethe conditioning disk 124 about the axis C when the conditioning disk124 is in contact with the polishing pad 104. In one embodiment, thesensor 138 may be disposed within the housing 136. In one embodiment,the sensor 138 may be an electrical current sensor coupled to the motor134. The electrical current sensor may detect changes in the currentdrawn by the motor 134 as the frictional forces between the conditioningdisk 124 and the polishing pad 104 change. In another embodiment, thesensor 138 may be a torque sensor, deflection sensor, or strain gauge,and may be positioned in the drive train between the motors and theconditioning head to measure forces on the drive train caused byfriction on the conditioning head.

A down force actuator 140 is utilized to urge the conditioning disk 124against the polishing pad 104. The down force actuator 140 is configuredto selectably set the force applied by the conditioning disk 124 on thepolishing pad 104. In one embodiment, the down force actuator 140 may bedisposed between the arm 128 and the shaft 126, or in other suitablelocations.

A down force sensor 142 is utilized to detect a metric indicative of thedown force of the conditioning disk 124 applied against the polishingpad 104. In one embodiment, the down force sensor 142 may be positionedin-line of the down force actuator 140, or may be placed in othersuitable locations.

In one embodiment, the polisher 100 may optionally include a padthickness sensor (not shown) coupled to the conditioning head 108. Thepad thickness sensor may be adapted to detect the thickness of apolishing pad disposed on the platen 102. The pad thickness sensor maybe utilized to determine an end of life of a polishing pad 104 disposedon the platen 102. In one embodiment, the pad thickness sensor may beutilized to further provide an additional feedback signal to thecontroller 152 to control the conditioning down force.

In general, the controller 152 is used to control one or more componentsand processes performed in the polisher 100. In one embodiment, thecontroller 152 may use sensory data as a feedback signal to control therate of material removed from the substrate 118 during processing. Thecontroller 152 may be coupled at various points to the polisher 100 inorder to transmit and receive signals from various components. Forexample, the controller 152 may transmit control signals to motors 112,120, 134, sweep actuator 144, and down force actuator 140 and receivesignals corresponding to forces detected by sensors 138, 142, 146, 148,150.

The controller 152 is generally designed to facilitate the control andautomation of the polisher 100 and typically includes a centralprocessing unit (CPU) 154, memory 156, and support circuits (or I/O)158. The CPU 154 may be one of any form of computer processors that areused in industrial settings for controlling various system functions,substrate movement, chamber processes, process timing and supporthardware (e.g., sensors, robots, motors, timing devices, etc.), andmonitor the processes (e.g., chemical concentrations, processingvariables, chamber process time, I/O signals, etc.). The memory 156 isconnected to the CPU 154, and may be one or more of a readily availablememory, such as random access memory (RAM), read only memory (ROM),floppy disk, hard disk, or any other form of digital storage, local orremote. Software instructions and data can be coded and stored withinthe memory for instructing the CPU 154. The support circuits 158 arealso connected to the CPU 154 for supporting the processor in aconventional manner. The support circuits 158 may include cache, powersupplies, clock circuits, input/output circuitry, subsystems, and thelike. A program, or computer instructions, readable by the controller152 determines which tasks are performable on a substrate. Preferably,the program is software readable by the controller 152 that includescode to perform tasks relating to monitoring, execution and control ofthe movement, support, and/or positioning of a substrate in the polisher100. In one embodiment, the controller 152 is used to control roboticdevices to control the strategic movement, scheduling and running of thepolisher 100 to make the processes repeatable, resolve queue timingissues and prevent over- or under-processing of the substrates.

In operation, as illustrated in FIG. 2, a polishing fluid 202 isprovided to the surface of the polishing pad 104 by the polishing fluiddelivery arm 190. The polishing head 106 urges the substrate 118 againstthe polishing pad 104. Contact with the surface of the rotatingpolishing pad 104 in the presence of the polishing fluid 202 planarizesthe surface of the substrate 118. In one embodiment, the polishing pad104 may be operated to planarize the substrate to remove material fromthe surface of the substrate at a pre-determined rate, sometimesreferred to as a removal rate, according to aspects described below.

Before, during and/or after planarizing the substrate 118, the polishingpad 104 may be conditioned. During conditioning, the conditioning head108 urges the conditioning disk 124 against the polishing pad 104 with apre-defined down force. The conditioning disk 124 rotates relative tothe surface of the polishing pad 104 while sweeping back and forthacross the polishing pad 104. Contact between the conditioning disk andpolishing pad provides the surface of the polishing pad with texturesuitable for maintaining a removal rate of the substrate. Theinteraction of the conditioning disk 124 with the polishing pad 104generates frictional forces which may be detected as described below.According to one embodiment, the removal rate of the polishing pad andfrictional forces exerted on the conditioning disk may be directlyrelated to the conditioning down force. As such, removal rate may beestimated based on the frictional forces observed for a particular downforce.

FIG. 3 is a flow diagram of a method 300 that may be performed using thepolisher 100 in accordance with aspects of the present disclosure. It iscontemplated that the method 300 that may be performed using othersuitable CMP systems and apparatuses.

At 302, the conditioning disk 124 is urged against the polishing pad 104using a down force. The down force of the conditioning disk 124 againstthe polishing pad 104 may be measured to provide feed-back to the forceactuator 140. In one embodiment, the down force is measured using ametric provided by the down force sensor 142. The measured down forcemay take into account the weight of the conditioning head 108.

At 304, a force required to move the conditioning disk 124 relative tothe polishing pad 104 is measured. The force required to move theconditioning disk 124 relative to the polishing pad 104 is definedherein as either a force or a torque. In one embodiment, the force ortorque required to sweep the conditioning disk across the polishing padis measured utilizing a metric providing by the sweep torque sensor. Thesweep torque may vary over time as a result of the change in resistivefrictional forces between the conditioning disk and the polishing pad asone or both of the conditioning disk and the polishing pad wear, and/orprocess conditions change. If the conditioning process is not changedover time, the measured sweep torque may decrease over the lifetime of aconditioning disk as the conditioning disk wears and its effective cutrate gradually reduces. As such, according to aspects described below,by using the sweep torque as a feedback signal, the down force may beadjusted to compensate for the worn conditioning disk, thereby improvingthe conditioning process to maintain a constant removal rate. In anotherembodiment, a frictional force between the conditioning disk 124 and thepolishing pad 104 generates a resistive force that may be detected bymonitoring changes in the forces required to rotate at least one of theconditioning disk 124 and/or the polishing pad 104. In anotherembodiment, a frictional force between the substrate 118 and thepolishing pad 104 generates a resistive force that may be measured byone or more of the sensors 138, 142, 146, 148 and 150 described above.It is understood that, in one embodiment, any of the above-describedfrictional forces may be used as a feedback signal to maintain aconstant removal rate.

At 306, the force measured at 304 may be compared to a model forceprofile (MFP) to determine a change in down force required to maintain auniform removal. In the case that torque is the force measured at 306,the MFP is a model torque profile (MTP). For the sake of brevity,reference to MFP here is intended MTP when applicable. In oneembodiment, the controller may compare the measured force or torque to amodel force profile to determine a change in down force. In oneembodiment, a new down force suitable to maintain a constant removalrate of material from the substrate may be calculated based on the MFP.In one embodiment, the new down force may be utilized during processingof the substrate as a close loop control routine. In another embodiment,the new down force value may be utilized for the next substrate to beprocessed as a feed back control routine. The MFP permits calculation ofthe new down force, given an amount of measured force and a desired rateof removal. In most applications, it has been determined that the MFPinvolved with maintaining a constant removal rate is non-linear, andthus, the change in torque is not linearly proportional to a change inthe down force. It is contemplated that for certain process conditions,the change in torque may be linearly proportional to a change in thedown force.

According to one embodiment, a new down force may be determined toimprove polishing pad lifetimes. Polishing pad wear may be lessened bydetermining a reduced initial down force that is nonetheless suitable toachieve a specified sweep torque target. Excessive polishing pad weardue to over-conditioning may be avoided by utilizing reduced initialdown force values when the conditioning disk is new, and the down forcemay be increased to maintain polishing performance as the surface of theconditioning disk wears.

At 308, the down force may be adjusted in response to the differencebetween the measured force and the MFP. In one embodiment, the downforce may be adjusted in response to the difference between the measuredtorque and a MFP. In one embodiment, a control signal may be transmittedto a down force actuator to increase the down force exerted on theconditioning disk. In another embodiment, a control signal may betransmitted to the down force actuator to decrease the down force toreduce excessive wear on the polishing pad and conditioning disk. It iscontemplated that a down force upper limit may be predefined to limitthe amount of the down force applied by the down force actuator.

The new down force may be adjusted while a substrate is currentlyundergoing processing, as a closed loop control routine, or may beadjusted for the next substrate to be processed, as a feedback controlroutine.

The MFP may be generated through empirical evidence, priorexperimentation and testing, modeling, calculating, or may be providedas a reference curve with the specification of the conditioning disk.Generally, it has been determined that the relationships between removalrate, sweep torque, and down force may evolve as a conditioning diskages. As the surface of the conditioning disk experiences greater wear,the removal rate consequently decreases for a fixed conditioning downforce. Accordingly, higher down force values would be used to providethe same level of disruption or regeneration of the polishing padsurface, and to maintain a constant removal rate, the conditioning downforce may be increased periodically.

According to certain aspects, it has also been determined that therelationship between sweep torque and conditioning down force may followa similar trend with continued use of a conditioning disk. For a fixedconditioning down force, the frictional forces between the conditioningdisk and polishing pad decreases as the abrasive surface of theconditioning disk wears, leading to a reduced torque experienced by asweep actuator. The reduction in sweep torque may indicate a decrease inpad conditioning effectiveness due to wear of the abrasive surface.Similarly, it has been determined that the relationship between removalrate and conditioning sweep torque also evolves over the life of theconditioning disk. A worn conditioning disk may less effectivelycondition a polishing pad which results in a reduced removal rate overtime.

According to certain aspects, a MFP may be developed using analysis oftwo different data sets. A first data set may be derived using a designof experiments performed using conditioning disks at different stages ofwear. The root mean square (RMS) of sweep torque may be measured forevery down force condition, along with a blanket substrate removal rate.A second dataset may be a marathon run of blanket substrates where thedown force was changed in a step-wise fashion, using a manual CLC. Inone embodiment, a down force applied may begin at 3 lb down force andmay increase to 11 lb down force over the course of processing 2,500substrates. The RMS of sweep torque may be measured on every substratewhile the blanket RR may be measured less frequently. These two datasets may be combined and a least squares estimation technique or anyother suitable data fitting technique may be used to estimate a modelforce profile between the RMS sweep torque (T), down force (DF), andblanket RR. In one embodiment, the structure of the model may be asfollows:

Log_(e)(T)=b*Log_(e)(RR)+a*Log_(e)(DF)  (1)

where a and b are constants obtained from a least squares estimation. Inone specific example, the values b and a calculated for an oxide CMPsystem utilizing a low down force conditioning arm manufactured byApplied Material, are 0.228 and 0.3, respectively. The constants b and amay be selected for specific pad materials, polishing fluids, substratematerial being polished, among other criteria.

The Equation 1 may also be rewritten as:

$\begin{matrix}{{{{{Log}_{e}(T)} - {{Log}_{e}({DF})}^{a}} = {{Log}_{e}({RR})}^{b}}{{{Log}_{e}\left( \frac{T}{{DF}^{a}} \right)} = {{Log}_{e}({RR})}^{b}}} & (2)\end{matrix}$

For a constant RR=k, the equation may be reduced as follows:

$\begin{matrix}{{{Log}_{e}\left( \frac{T}{{DF}^{a}} \right)} = {{Log}_{e}k}} & (3) \\{{\left( \frac{T}{{DF}^{a}} \right) = k_{1}},{or}} & (4) \\{T = {k_{1}*{DF}^{a}}} & (5)\end{matrix}$

Equation 5 illustrates a model force profile for a target sweep torqueas a function of down force to achieve a constant removal rate.

Thus, a methodology has been provided to maintain constant removal ratesover the life of a conditioning disk. Embodiments of the inventionadvantageously compensate for loss of conditioning effectiveness tomaintain a desired process performance. The methodology may be utilizedin-situ as a running process, or as a feedback routine to substantiallyeliminate process drift. Additionally, embodiments of the inventionadvantageously extend the useful lifetime of polishing pads bydetermining a reduced down force still sufficient to achieve a specifiedsweep torque target. Consequently, embodiments of the inventionadvantageously reduce excessive polishing pad wear due toover-conditioning, for example, when the conditioning disk is new andfreshly abrasive. According to certain embodiments, the pad lifetime maybe improved by 20% to 60% over conventional approaches. Likewise, it isunderstood that embodiments of the invention advantageously increase theuseful lifetime of conditioning disks.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow. For example, while the foregoingis directed to embodiments of the present invention focused on a sweeptorque of a conditioning disk, it is contemplated that torques measuredthroughout a CMP system, such as at other motors or actuators, may beused to determine a model force profile for controlling the polisher.

What is claimed is:
 1. A method for conditioning a polishing pad,comprising: applying a down force to a conditioning disk that urges theconditioning disk against the polishing pad; measuring a torque requiredto sweep the conditioning disk across the polishing pad; determining achange in down force by comparing the measured torque to a model forceprofile (MFP); and adjusting the down force that the conditioning diskapplies against the polishing pad in response to the determined change.2. The method of claim 1, wherein the MFP comprises an estimatedrelationship between the down force, the torque, and a rate of removalof material from a substrate by the polishing pad.
 3. The method ofclaim 1, wherein the determining comprises: determining the change indown force necessary to maintain a rate of material removal from asubstrate by the polishing pad.
 4. The method of claim 1, wherein thedetermining comprises: determining a decrease in down force necessary toachieve a target torque of the MFP.
 5. The method of claim 1, whereinthe determining comprises: determining an increase in down forcenecessary to maintain a constant removal rate of the polishing pad inresponse to the measured torque being less than the MFP.
 6. The methodof claim 1, wherein the adjusting the down force comprises modifying aconditioning recipe for the polishing pad.
 7. The method of claim 1,wherein the adjusting the down force comprises adjusting the down forcein-situ during conditioning of the polishing pad.
 8. A method forconditioning a polishing pad, comprising: applying a down force to urgea conditioning disk against the polishing pad; measuring a frictionalforce generated by the conditioning disk contacting the polishing pad;comparing the measured frictional force to a model force profile (MFP)to determine a change in down force; and adjusting the applied downforce in response to the comparison between the measured frictionalforce and the MFP.
 9. The method of claim 8, wherein the MFP furthercomprises: an estimated relationship between the down force, thefrictional force, and a rate of removal of material from a substrate bythe polishing pad.
 10. The method of claim 9, wherein the frictionalforce comprises a rotational torque required to rotate at least one of aconditioning head, a platen, or a polishing head.
 11. An apparatus forconditioning a polishing pad with a conditioning disk, comprising: aplaten adapted to support the polishing pad; a conditioning head adaptedto retain the conditioning disk; a down force actuator operable to movethe conditioning head in a manner that the conditioning head applies adown force against the polishing pad with; an arm coupled to theconditioning head to support the conditioning head above the platen; anda sweep actuator coupled to the arm and operable to sweep theconditioning head across the platen; and a sweep torque sensor operableto measure a torque required to sweep the conditioning head across theplaten when the conditioning disk is in contact with the polishing pad.12. The apparatus of claim 11, further comprising: a controller coupledto the down force actuator and operable to instruct the down forceactuator to apply the down force at a value selected in response to amodel force profile (MFP)
 13. The apparatus of claim 12, wherein thecontroller is operable to provide a closed loop control of down forceapplied by the down force actuator based on the torque measured by thesweep torque sensor to maintain a substantially constant removal rate ofmaterial from a substrate by the polishing pad.
 14. The apparatus ofclaim 11, further comprising: a pad thickness sensor coupled to theconditioning head.